Learn and practice the Hash Maps and Knowing What to Track patterns to solve coding interview problems.

Speedrun the Apple Coding Interview: Top Problems in JavaScript

Hash maps are powerful data structures that enable programmers to design more efficient solutions for certain problems. After practicing solving problems using hash maps, we will move to the more general problem-solving skill of figuring out the key characteristics of a problem, using various techniques to track them, in order to design remarkably efficient solutions for seemingly complex problems.

Hashing and Tracking


# Statement

Given the two distinct integer arrays, `nums1` and `nums2`, where `nums1` is a subset of `nums2`, find all the next greater elements for `nums1` values in the corresponding places of `nums2`.

In general, the next greater element of an element, $x$, in an array is the first greater element present on the right side of $x$ in the same array. However, in the context of this problem, for each element $x$ in `nums1`, find the next greater element present on the right side of $x$ in `nums2`  and store it in the `ans` array. If there is no such element, store $-1$ for this number.  The `ans` array should be of the same length as `nums1`, and the order of the elements in the `ans` array should correspond to the order of the elements in `nums1`. 

Return the `ans` array after finding the next greater elements. 

> **Note:** The input data may or may not be sorted.

**Constraints:**

- $1 \leq$ `nums1.length` $\leq$ `nums2.length` $\leq 10^3$
- $0 \leq$ `nums1[i]`, `nums2[i]` $\leq 10^4$ 
- `nums1` have distinct integers.
- `nums2` have distinct integers.
- All integers in `nums1` also appear in `nums2`.

# Examples

# Understand the problem

Let’s take a moment to make sure you've correctly understood the problem. The quiz below helps you check if you're solving the correct problem:

What is the output if the following arrays are given as input?

nums1 = [5, 4, 7]

nums2 = [4, 5, 7, 3]



What is the output if the following arrays are given as input?
nums1 = [5, 4, 7]
nums2 = [4, 5, 7, 3]

What is the output if the following arrays are given as input?

nums1 = [9, 7, 6]

nums2 = [5, 7, 6, 8, 9]


What is the output if the following arrays are given as input?
nums1 = [9, 7, 6]
nums2 = [5, 7, 6, 8, 9]

What is the output if the following arrays are given as input?

nums1 = [25, 56, 100, 63]

nums2 = [63, 25, 56, 76, 100]


What is the output if the following arrays are given as input?
nums1 = [25, 56, 100, 63]
nums2 = [63, 25, 56, 76, 100]

Next Greater Element

Next Greater Element

# Quick Fix\nYou are Edward, a Senior Software Engineer who specializes in coding interview patterns, data structures and algorithms, and is great at debugging issues in code.\n\n- Edward, please keep in mind that you do not have to print the data of the placeholders. Also, do not mention anything about yourself.\n\nEdward, you are given the following data in the placeholders:\n- Function name: `nextGreaterElement`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `JavaScript`\n\n- Edward, your task is to understand the type of error that is occurring in the coding language provided.\n- Once you've understood the error as per the error’s description, and why the test case failed, you will suggest how the candidate can improve their solution code to fix the error.\n- Edward, you will not give the corrected code, just a hint on what the error tells, and how to fix it.\n- Edward, please limit your response strictly to 100 words. No more words than the limit.

# Review Code\nEdward, your current task is to review the `nextGreaterElement` function(s) and the other function(s) invoked in it/them.\n\n-Take your time and review the candidate's solution code.\n- Do not review any extra/helper files or functions not invoked in the solution function `nextGreaterElement`.\n- Do not review the code of data structure objects instantiated like a binary tree, linked list, trie, union find, etc., in the solution functions. Just review the solution/algorithm function of the problem. \n- Also, if the solution code doesn’t use the coding pattern stated above, then please encourage the candidate to reattempt the problem using that pattern corectly.

# Review time complexity\n- Please provide a brief time complexity analysis of the function(s) `nextGreaterElement`. \n- While calculating time complexity, include the time complexity of built-in JavaScript functions invoked by the code. \n- The answer should be to the point in approximately 60 words.

# Review space complexity\n- Please provide a brief space complexity analysis of the function(s) `nextGreaterElement`. \n- While calculating space complexity, include the space complexity of built-in JavaScript functions invoked by the code. \n- Do not include the space used for the input parameters and the space used for just returning an output (not used as a scratch space). \n- The answer should be to the point in approximately 60 words.

def evaluate_results(self) -> object:
        """
        self.__inputs is the list of inputs that are passed as input to function
        self.__actual_output is a list of outputs that is produced by the learner's code
        return : self.__edu__result object
        """
        def next_greater_element(nums1, nums2):
            stack = []      
            map = {}        

            for current in nums2:
                while stack and current > stack[-1]:
                    map[stack.pop()] = current
                stack.append(current)

            while stack:
                map[stack.pop()] = -1

            ans = []

            for num in nums1:
                ans.append(map[num])
                
            return ans

        # multiple outputs will be added at each index respectively for a single test_case
        expected_outputs = [] 
        # store the 1st expected output that should be displayed to the user 
        expected_outputs.append(next_greater_element(self.__inputs[0] , self.__inputs[1]))

        if json.dumps(next_greater_element(self.__inputs[0] , self.__inputs[1])) == json.dumps(self.__actual_outputs[0]):
            self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
        else:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

        return self.__edu_result

function nextGreaterElement(nums1, nums2){

    // Replace this placeholder return statement with your code
    return [];
}

export {
    nextGreaterElement
}

next_greater_element (1).yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : [2, 4]
    - 2 : [1,2,3,4]
    output: 
    - 1 : [3, -1]
  - name: "testcase 2"
    inputs:
    - 1: [1, 3]
    - 2: [1, 2, 3, 4]
    output:
    - 1 : [2, 4]
  - name: "testcase 3"
    inputs:
    - 1 : [1,10]
    - 2 : [1,3,5,10]
    output: 
    - 1 : [3, -1]
  - name: "testcase 4"
    inputs:
    - 1: [15, 13]
    - 2: [10,12,13,15,9]
    output:
    - 1 : [-1, 15]
  - name: "testcase 5"
    inputs:
    - 1: [137,59,92,122,52,131,79,236]
    - 2: [137,59,92,122,52,131,79,236]
    output:
    - 1 : [236,92,122,131,131,236,236,-1]

def sum(input_1, input_2):
    return input_1+input_2

def validate_inputs(self) -> None:
 """
 self.__inputs is the list of inputs that are passed as input to the function
 store the result using the self.store_result(error_msg, status)
 return: void
 """
 nums1_length = len(self.__inputs[0])
 nums2_length = len(self.__inputs[1])
 def is_valid_1d_array(arr):
 if not isinstance(arr, list):
 return False # Not a valid 1D array if it's not a list
 
 return all(isinstance(elem, int) and not isinstance(elem, bool) for elem in arr)
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_1d_array(self.__inputs[0]) or not is_valid_1d_array(self.__inputs[1]):
 self.store_result(("Invalid test case."), ValidationFunction.FAIL)
 elif not (1 <= nums1_length <= 1000):
 self.store_result("Input 1 length must be between 1 and 1000", ValidationFunction.FAIL)
 elif not (1 <= nums2_length <= 1000):
 self.store_result("Input 2 length must be between 1 and 1000", ValidationFunction.FAIL)
 elif not (1 <= nums1_length <= nums2_length):
 self.store_result("Length of 'nums1' should be less than or equal to length of 'nums2'", ValidationFunction.FAIL)
 # elif not (nums1_length <= nums2_length <= 1000):
 # self.store_result("Length of Input 2 must be between and the length of Input 2", ValidationFunction.FAIL)
 elif any(num > 10000 for num in self.__inputs[0]) or any(num < 0 for num in self.__inputs[0]) or any(num > 10000 for num in self.__inputs[1]) or any(num < 0 for num in self.__inputs[1]):
 self.store_result("All integers should be between 0 and 10^4", ValidationFunction.FAIL)
 elif nums1_length != len(set(self.__inputs[0])) or nums2_length != len(set(self.__inputs[1])):
 self.store_result("Input lists should have distinct integers", ValidationFunction.FAIL)
 elif not set(self.__inputs[0]).issubset(set(self.__inputs[1])):
 self.store_result("All integers in nums1 should also appear in nums2", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Next Greater Element I problem.

Hash Maps

Knowing What to Track

Conclusion

Next Greater Element

Statement

Examples

Understand the problem

Figure it out!

Try it yourself